Abstract : The image irradiance equation constrains the relationship between surface orientation in a scene and the irradiance of its image. Additional constraints, needed to recover surface orientation from image irradiance, usually require the recovered surface to be smooth. We demonstrate that smoothness is not sufficient for this task. The 'beliefs' a visual system has about the laws of surface radiance provide sufficient constraints to relate the irradiance of an image to the corresponding scene's surface orientations. We propose a set of beliefs and derive surface orientation - image irradiance equations for a visual system with these beliefs. The surface orientations, derived from the image by a visual system with these beliefs, may not be those present in the scene, rather they are those believed to be present. (Author)